Small universal and permanent magnet motors typically use a cartridge type brush assembly that rides on the commutator and electrically connects the motor power supply to the armature through the commutator. The assembly comprises a tubular holder and a brush axially slidable in the holder and protruding through a mouth at one end of the holder to engage a motor commutator. The brush is biased toward the commutator by a helical coil spring which has the coils at one end surrounding a neck of the brush. Such brush assemblies have been found to be subject to chattering (i.e., vibrating or bouncing) of the brush on the commutator responsive to commutator rotation. Chattering is objectional because it generates electrical noise and reduces brush life due to arcing between the brush and commutator. Also, poor and uneven contact of the brush on the commutator can cause uneven wear and grooving of the commutator. This can reduce the life of the currently installed brush and also of replacement brushes because of a mismatch between the new brush shape and the unevenly worn commutator surface. An unevenly worn commutator can further cause a new brush to burn the commutator (due to arcing) because of an extreme mismatch between the brush and commutator surfaces.
Another problem that results from poor contact between the brush and commutator occurs particularly in portable power tools such as circular saws which use a dynamic motor brake such as disclosed in U.S. Pat. No. 5,063,319. Two important criteria for evaluating brake performance are certainty of the application of the brake and the time required for the brake to completely stop blade rotation. Over the life of the brushes, poor contact can cause brake actuation to be less reliable and braking time to vary.
As disclosed, e.g., in U.S. Pat. Nos. 2,474,601 and 2,615,939, chattering of the brush can be mitigated by providing a force normal to the axis of the brush and acting in the direction of rotation of the commutator to hold the brush against the side of the brush holder. Such a force can be provided by beveling the rear face (engaged by the helical spring) of the brush so that the face is inclined with respect to the brush axis and the force exerted by the spring includes both an axial and normal components relative to the holder axis. However, because of the criticality of achieving low levels of chattering particularly in products with dynamic brakes, further improvements in the reduction of chattering than have been achievable with prior art systems are desirable.